KR102230103B1 - High-strength steel sheet having excellent formability and its manufacturing method - Google Patents

Abstract

Cold rolled and heat treated steel sheet is 0.17% ≤ carbon ≤ 0.24%, 1.9% ≤ manganese ≤ 2.2%, 0.5% ≤ silicon ≤ 1%, 0.5% ≤ aluminum ≤ 1.2%, where Si + Al ≥ 1.3%, 0.05% ≤ chromium ≤ 0.2%, 0.015% ≤ niobium ≤ 0.03%, sulfur ≤ 0.003%, phosphorus ≤ 0.03% and optionally 0.005% ≤ titanium ≤ 0.05%, 0.001% ≤ molybdenum ≤ 0.05%, the balance being iron And inevitable impurities resulting from smelting, and the microstructure of the coated steel sheet is, by area fraction, 10 to 20% of retained austenite, 40 to 55, in which the austenite phase has a carbon content of 0.9 to 1.1%. % Polygonal ferrite, 15 to 40% granular bainite and at least 5% tempered martensite, the sum of tempered martensite and retained austenite is 20 to 30%.

Description

High-strength steel sheet having excellent formability and its manufacturing method

The present invention relates to a steel sheet having excellent mechanical properties suitable for use in the manufacture of automobiles, and a method of manufacturing the same, and in particular, the present invention provides high strength and high formability.

In recent years, as the emphasis on fuel economy and carbon emissions increases in terms of global environmental conservation, reduction in vehicle weight is necessary; As a result, there is a need for the development of steel sheets with higher strength, elongation and acceptable mechanical properties. Therefore, automotive steel parts are required to satisfy two properties that are generally considered difficult to obtain together, such as high formability and ductility on the one hand and high tensile strength on the other.

Through intensive research and development efforts, the strength of the material was increased to reduce the vehicle weight. Conversely, when the strength of the steel sheet increases, the formability decreases, and therefore, it is necessary to develop a material having high strength and high formability.

Therefore, high-strength steel having excellent formability such as TRIP (Transformation Induced Plasticity) steel has been developed. TRIP steel provides a good balance between mechanical strength and formability due to its complex structure containing austenite that gradually deforms into strain. TRIP steels may also contain components such as martensite and austenite (MA) and islands of bainite, and ferrite, which is a ductile component. The TRIP steel has a very high reinforcing ability, so that the deformation can be well dispersed in the event of a collision or during the molding of automobile parts. Thus, it is possible to manufacture parts that are as complex as those made from conventional steel but with improved mechanical properties, which in turn make it possible to reduce the thickness of the parts to conform to the same functional specifications in terms of mechanical performance. Therefore, this steel is an effective countermeasure to the requirements of reducing vehicle weight and increasing safety. In the field of hot rolled or cold rolled steel sheets, this type of steel has application in particular for structural and safety parts for automobiles. Various attempts have been made to produce a high strength and high formability steel grade and a high strength and high formability steel sheet by imparting high strength and high formability to steel.

US9074272 describes a steel with a chemical composition of 0.1-0.28% C, 1.0-2.0% Si, 1.0-3.0% Mn and the balance consisting of iron and unavoidable impurities. The microstructure contains 9 to 17% of retained austenite, 40 to 65% of bainite-based ferrite, 30 to 50% of polygonal ferrite and less than 5% of martensite. It refers to a cold rolled steel sheet with good elongation, but the invention described in US9074272 does not achieve the tensile strength of 900 MPa, which is now required for many structural automotive parts.

US 2015/0152533 Silver C: 0.12-0.18%, Si: 0.05-0.2%, Mn: 1.9-2.2%, Al: 0.2-0.5%, Cr: 0.05-0.2%, Nb: 0.01-0.06%, P: 0.02 % Or less, S: 0.003% or less, N: 0.008% or less, Mo: 0.1% or less, B: 0.0007% or less, Ti: 0.01% or less, Ni: 0.1% or less, Cu: 0.1% or less, the balance Disclosed is a method of manufacturing a high-strength steel composed of iron and unavoidable impurities. The steel sheet has a microstructure consisting of 50-90% by volume of ferrite (including bainite-based ferrite), 5-40% by volume of martensite, up to 15% by volume of retained austenite and up to 10% by volume of other structural components. . Even though the steel disclosed in US2015/0152533 contains a significant amount of martensite (ie, 40% or less), the steel does not achieve a tensile strength level of 900 MPa.

In addition, document JP 2001/254138 describes a steel having a chemical composition of 0.05-0.3% C, 0.3-2.5% Si, 0.5-3.0% Mn, and 0.001-2.0% Al, and the balance consisting of iron and unavoidable impurities. have. This structure contains residual austenite with a mass concentration of at least 1% carbon and a volume fraction of 3 to 50% and ferrite in an amount of 50 to 97%. This invention cannot be used to manufacture steels that require specific mechanical strengths associated with high ductility to form automotive composite structural parts.

In addition, EP2765212 has a microstructure in which the area ratio of martensite is 5-70%, the area ratio of retained austenite is 5-40%, the area ratio of bainite ferrite in the upper bainite is 5% or more, and the total is 40% or more, and 25% of martensite. The above is a tempered martensite and proposes a high-strength steel sheet having excellent ductility and elongation flangeability having a polygonal ferrite area ratio of more than 10% and less than 50%.

Accordingly, in view of the aforementioned documents, it is an object of the present invention to provide a steel sheet that makes it possible to obtain a greater weight reduction with a capacity to fit current automotive manufacturing practices for manufacturing complex automotive parts and members.

It is an object of the present invention to solve these problems by producing a usable cold-rolled steel sheet having the following at the same time:

-A maximum tensile strength TS of not less than 980 MPa, preferably more than 1050 MPa or more than 1100 MPa,

-Yield strength greater than 550 MPa,

-A yield ratio of 0.60 or more,

-A total elongation TE of at least 17%, preferably more than 19%,

-Hole expansion ratio of 18% or more (measured according to ISO standard 16630:2009).

Preferably, these steels have good suitability for forming, in particular rolling, good weldability and good coatability.

Another object of the present invention is to produce a steel having excellent resistance to liquid metal embrittlement cracking.

Another object of the present invention is to make available a method of making a sheet that is compatible with conventional industrial applications without being too sensitive to slight small changes in manufacturing parameters.

1 is a micrograph showing the microstructure of the steel of the present invention; The tempered martensite and austenite appear as hazy constituents, and the remainder is ferrite and granular bainite.
FIG. 2A shows the homogeneous distribution of tempered martensite in the steel sheet of the present invention, and FIG. 2B shows the heterogeneous distribution of martensite in the reference steel sheet.

The steel sheet according to the present invention exhibits a specific composition described below.

Carbon is present in the steel of the present invention from 0.17% to 0.24%. Carbon plays an important role in the formation of microstructures through the TRIP effect and in strength and ductility: it is not possible to obtain a significant TRIP effect when the carbon content is less than 0.17%. If it exceeds 0.24%, weldability deteriorates. The carbon content advantageously comprises 0.20 to 0.24% so as to obtain high strength and high elongation at the same time.

Manganese is added in the range of 1.9% to 2.2% in current steels. Manganese is an element that provides hardening by solid solution substitution in ferrite. A minimum content of 1.9% by weight is required to obtain the required tensile strength. Nevertheless, if it exceeds 2.2%, manganese delays the formation of bainite and further enhances the formation of austenite with a reduced amount of carbon that is transformed into martensite rather than residual austenite in a later step, which It is harmful to the required properties.

Silicon is added to the steel of the present invention in an amount of 0.5% to 1%. Silicon plays an important role in the formation of microstructures by slowing the precipitation of carbides during the leveling step following primary cooling, which makes it possible to concentrate carbon in the austenite for stabilization. Silicon plays an effective role in combination with aluminum and the best results are obtained at content levels above 0.5% with regard to certain properties. However, if silicon is added in an amount greater than 1%, the melt coatability is adversely affected by promoting the formation of oxides adhering to the surface of the product, thereby reducing weldability. This can also lead to liquid metal embrittlement due to liquid Zn penetration into the austenite grain boundaries during spot welding. A content of less than 1% simultaneously provides not only good coatability but also very good suitability for welding. The silicon content may preferably be 0.7 to 0.9% in order to limit the formation of brittle martensite instead of bainite.

Aluminum plays an important role in the present invention by significantly slowing the precipitation of carbides and stabilizing residual austenite. This effect is obtained when the aluminum content is included in the range of 0.5% to 1.2%. Preferably, the aluminum content may be 0.7% or more and 0.9% or less. In addition, it is generally thought that high levels of Al increase the corrosion of refractory materials and there is a risk of clogging the nozzles during casting steel upstream of rolling. Also, aluminum can cause macro segregation by negative segregation. In the case of excess, aluminum reduces hot ductility and increases the risk of defects appearing during continuous casting. If the casting conditions are not carefully controlled, central segregation occurs in the annealed steel sheet due to micro and macro segregation defects. This central band is harder than the surrounding matrix and adversely affects the formability of the material.

In addition to the individual limitations mentioned above, the sum of aluminum and silicon should be at least 1.3%, preferably at least 1.4%, because both elements synergistically contribute to the stabilization of retained austenite during the annealing cycle, more specifically bays. This is because it significantly slows the precipitation of carbides during the night transformation. This makes it possible to obtain enrichment of austenite by carbon, resulting in stabilization at room temperature in the steel sheet.

In addition, the inventors of the present invention, when Si/10> 0.30%-C (Si and C are expressed in wt%), due to LME (liquid metal embrittlement phenomenon), silicon is spot welding of the coated sheet In particular, it has been found to be harmful to galvanized or alloyed or electrogalvanized sheets. When LME occurs, cracks occur in the grain boundaries in the weld metal of the weld joint and in the heat affected zone. Therefore, (C + Si/10) should be kept below 0.30%, especially when the sheet is to be coated.

The inventors have also found that in order to reduce the occurrence of LME, the Al content should be 6(C+Mn/10)-2.5% or more for the domain of the composition considered.

Chromium is added to the steel of the present invention in an amount of 0.05% to 0.2%. Like manganese, chromium increases the hardenability that promotes martensite formation. If the chromium content is higher than 0.05%, it is useful to reach the required tensile strength. However, if the chromium content is higher than 0.2%, the bainite formation is delayed, and thus the austenite is not sufficiently concentrated in carbon during the equalization step; In practice this austenite transforms somewhat entirely into martensite during cooling to ambient temperature, and the elongation is too low. Therefore, the chromium content is 0.05 to 0.2%.

Niobium is added to the steel of the present invention in an amount of 0.015 to 0.03 to induce the formation of carbonitrides to impart strength by precipitation hardening. Since niobium delays recrystallization during heating, the microstructure formed at the end of the holding temperature and consequently after full annealing becomes finer, leading to hardening of the product. However, when the niobium content exceeds 0.03%, a large amount of carbonitrides is formed, and there is a tendency to lower the ductility of the steel.

Titanium is any element that can be added to the steel of the present invention in an amount of 0.005% to 0.05%. Like niobium, titanium precipitates to form carbonitrides and contributes to hardening. However, titanium is also involved in the formation of large TiN that appears during solidification of the cast product. Therefore, the amount of titanium is limited to 0.05% in order to avoid coarse TiN which is detrimental to hole expansion. When the titanium content is added below 0.005%, it does not have any effect on the steel of the present invention.

Molybdenum is any element that can be added to the steel of the present invention in an amount of 0.001% to 0.05%. Molybdenum can play an effective role in improving hardenability, delaying bainite formation, and avoiding carbide precipitation in bainite. However, since the addition of molybdenum excessively increases the cost of adding the alloying element, its content is limited to 0.05% for economic reasons.

The sulfur content of the present invention should be kept as low as possible; Therefore, the content of sulfur in the present invention is 0.004% or less. Sulfur content of 0.004% or more reduces ductility due to the excessive presence of sulfides such as MnS (manganese sulfide), which reduces the workability of the steel and also causes cracking.

Phosphorus may be present in the steel of the present invention in an amount up to 0.03%. Phosphorus is an element that hardens in solid solution but greatly reduces its suitability for high temperature ductility and spot welding. For this reason, in order to obtain good high temperature ductility and good suitability for spot welding, the phosphorus content should be limited to 0.03%.

The steel sheet of the present invention exhibits a specific microstructure comprising several phases whose quantity is given as an area fraction.

The polygonal ferrite component imparts improved elongation to the steel of the present invention and ensures elongation and hole expansion ratio at the required level. Polygonal ferrite is a soft and essentially ductile component. It can be distinguished from regular ferrites formed in the cooling step because of its low solid solution carbon content and very low dislocation density. Polygonal ferrites should be present in an amount of at least 40% and at a level of up to 55%. Polygonal ferrite imparts elongation to the invention because of its softness compared to other hard phases present such as tempered martensite and because of the very limited amount of carbon present in the polygonal ferrite, which can be as low as 0.005%. In addition, since the dislocation density is low, it contributes to the hole expansion ratio. This polygonal ferrite is mainly formed during heating and holding at a temperature equivalent to an intercritical annealing. A certain amount of regular ferrite can be formed during cooling, but because of the manganese content, the regular ferrite content that appears in the cooling step is always less than 5%.

The granular bainite present in the steel of the present invention is distinguished from the conventional bainite structure because the granular bainite of the present invention has a very low carbide density. Here, a low carbide density means 100 or less carbides per area unit ^{of 100 μm 2.} Since the dislocation density is high (around 10 ¹⁵ /m ^-2 ), this granular bainite imparts high strength to the steel of the present invention, as opposed to polygonal ferrite. The amount of granular bainite is 15 to 40%.

Retained austenite is present as a component in an amount of 10 to 20%, and is an essential component to ensure the TRIP effect. The retained austenite of the present invention has a carbon percentage of 0.9 to 1.1%, which plays an important role in stabilizing the austenite at room temperature and improving the TRIP effect, which provides suitable moldability for the present invention. In addition, the carbon-rich residual austenite contributes to the formation of granular bainite because the solubility of carbon in the austenite is high, which delays the formation of carbides in the bainite. In a preferred embodiment, the average particle size of this retained austenite is less than 2 μm. Retained austenite is measured by a magnetic method called sigmametry, which, unlike other ferromagnetic phases, consists of measuring the magnetic moment of the steel before and after heat treatment that makes paramagnetic austenite unstable.

In addition, the steel of the present invention contains at least 5% of tempered martensite, which is a component composed of fine laths elongated in one direction within each particle generated from primary austenite particles, < Fine steel carbides are deposited between the laths along the 111> direction. This tempering of martensite allows to increase the yield strength due to the decrease in the hardness gap between martensite and ferrite or bainite, and for the same reason and due to the reduction of martensite increases the rate of hole expansion. The total content of tempered martensite and retained austenite is 20 to 30%, preferably 25 to 30%. The tempered martensite and austenite may exist in the form of martensite-austenite islands or may exist individually in the form of separate microstructures. The present steel does not contain any untempered martensite, since the untempered martensite is a hard phase and thus reduces the yield strength of the steel and also reduces the formability of the steel of the present invention.

In a preferred embodiment of the invention, the homogeneity of the distribution of the tempered martensite content is characterized in the following way: The tempered martensite fraction (TM) is measured in an arbitrary area of ^{50×50 μm 2 in the steel sheet and} Compared to the average fraction (TM ^{* ).} The distribution of tempered martensite is |(TM)-(TM ^* )| If ≤ 1.5%, it is defined as homogeneous. This homogeneous distribution improves the hole expansion rate.

The steel sheet according to the invention can be produced by any suitable process. However, it is preferable to use the process described below.

Casting of semi-finished products can be carried out in the form of ingots or in the form of thin slabs or thin strips, the thickness of which ranges from about 220 mm for slabs to tens of millimeters for thin strips or slabs.

For simplicity, the description below will focus on the slab as a semi-finished product. Slabs having the above-described chemical composition are produced by continuous casting and are provided for further processing according to the production method of the present invention. Here, the slab may be used at a high temperature during continuous casting, or it may be cooled to room temperature first and then reheated.

The temperature of the slab to be hot-rolled should preferably be at least the Ac3 point, at least 1000°C, and at least 1280°C. The temperatures mentioned here are specified to ensure that the austenite range is reached at all points in the slab. When the temperature of the slab is lower than 1000°C, an excessive load is applied to the rolling mill, and the temperature of the steel may also be lowered to the ferrite transformation temperature during rolling. Therefore, to ensure that the rolling is in the complete austenite zone, reheating has to be carried out at 1000°C or higher. In addition, the temperature should not exceed 1280° C. in order to avoid inappropriate growth of austenite particles, which results in coarse ferrite particles and reduces the recrystallization ability of the particles during hot rolling. In addition, temperatures above 1280° C. increase the risk of formation of harmful thick layer oxides during hot rolling. The finish rolling temperature should be 850℃ or higher. It is desirable to have a finish rolling temperature higher than the Ar3 point to ensure that the steel to be hot rolled is rolled in the complete austenite zone.

Subsequently, the hot-rolled steel sheet obtained in this way is cooled to a coiling temperature of 580°C or less at a cooling rate of 35 to 55°C/s to obtain the essential microstructure of the present invention, because this range of cooling rates is This is because it helps in the formation of bainite. The cooling rate should not exceed 55°C/s to avoid excessive formation of martensite. The coiling temperature should be 580℃ or less, because if it is higher than this temperature, there is a risk that micro-segregation and intergranular oxidation may intensify. The preferred winding temperature of the hot-rolled steel sheet of the present invention is 450 to 550°C.

Subsequently, the hot-rolled steel sheet is cooled to room temperature, preferably at a cooling rate of 125° C./h or less.

Thereafter, pickling is performed on the hot-rolled steel sheet to remove scale, and the hot-rolled sheet is cold-rolled, typically with a thickness reduction of 30 to 90%.

The cold rolled steel sheet obtained in the cold rolling process is subjected to critical annealing and subsequent other heat treatment processes to impart the mechanical properties and microstructure required to the steel of the present invention.

The cold rolled steel sheet has a heating rate of 1 to 20°C/s, preferably greater than 2°C/s so that the ratio of ferrite to austenite is from 60:40 to 35:65, and from Ac1 to Ac3, preferably from 780 to It is annealed continuously to a soaking temperature of 950°C. The cracking is preferably carried out for more than 10 seconds and should not be more than 600 seconds.

The sheet is then cooled to a bainite temperature transformation range of 440 to 480° C. at a rate higher than 25° C./s, whereby a cooling rate of 30° C./s or more is preferred. While not wishing to be bound by theory, the inventors believe that the homogeneity of martensite formation is primarily due to this high cooling rate after annealing.

The steel sheet is then held at this temperature for 20 to 250 seconds, preferably 30 to 100 seconds, to cause bainite formation. If the cold-rolled steel sheet is kept for less than 20 seconds, the amount of bainite is too small and the concentration of austenite is insufficient, so that the amount of retained austenite becomes less than 10%. If it exceeds 250 seconds, it causes precipitation of carbides in the bainite, resulting in depletion of austenite in the carbon before final cooling. Holding at 440 to 480°C is carried out to form granular bainite and promote austenite concentration in carbon.

Subsequently, hot dip galvanization (GI) is carried out by immersing in a zinc or zinc alloy bath, the temperature of which may be 440 to 475°C, and then the GI product is 1 to 20 to obtain residual austenite and limit the martensite content. It is cooled to room temperature at a cooling rate of °C/s, preferably 5 to 15 °C/s.

Then, the galvanized steel sheet is subjected to a batch annealing treatment. During this batch annealing, the galvanized steel sheet is heated to a temperature of 170 to 350°C, preferably 170 to 250°C for 12 to 250 hours, preferably 12 to 30 hours, and then cooled to room temperature. This is done to effectively temper fresh martensite.

Yes

The following tests, illustrations, figurative examples and tables presented herein are non-limiting in nature and should be considered for illustrative purposes only, and represent advantageous features of the invention, and the importance of the process parameters selected by the inventors after large-scale experiments. We will explain and establish the properties that can be achieved by the steel of the present invention.

The composition of the steel sheets of the test samples is in Table 1, where the steel sheets are each manufactured according to the process parameters gathered in Table 2. Table 3 shows the obtained microstructure, and Table 4 shows the evaluation results of the use characteristics.

It should be emphasized that due to the difference in measurement methods, the value of the hole expansion ratio HER according to the ISO standard is very different from the value of the hole expansion ratio λ according to JFS T 1001 (Japan Iron and Steel Federation standard) and cannot be compared. Tensile strength TS and total elongation TE are measured according to ISO standard ISO 6892-1 published in October 2009. Due to the difference in the measurement method, in particular due to the difference in the geometric shape of the specimens used, the value of the total elongation TE measured according to the ISO standard is very different from the value of the total elongation measured according to the JIS Z 2201-05 standard, Especially lower than that.

Table 1-Steel composition

Table 1 shows steels whose composition is expressed as a weight percentage. The steel compositions I1 to I6 are for the production of the sheet according to the invention, and this table also defines the reference steel composition indicated by R1 to 9 in the table.

[Table 1]

Table 2-Process parameters

Table 2 shows in detail the annealing process parameters implemented in the steel samples shown in Table 1. Table 1 also shows a table of bainite transformation temperatures of the inventive and reference steels. The calculation of the bainite transformation temperature is carried out using:

Bs=839-(86*[Mn]+23*[Si]+67*[Cr]+33*[Ni]+75*[Mo])-270*(1-EXP(-1,33*[C ]))

Ac1 is calculated using the formula described in "Darstellung der Umwandlungen fur technische Anwendungen und Moglichkeiten ihrer Beeinflussung, H.P. Hougardy, Werkstoffkunde Stahl Band 1, 198-231, Verlag Stahleisen, Dusseldorf, 1984":

Ac1 = 739-22*C-7*Mn + 2*Si + 14*Cr + 13*Mo-13*Ni.

In this formula, Ac1 is in degrees Celsius, and C, Mn, Si, Cr, Mo and Ni are the weight percent of C, Mn, Si, Cr, Mo and Ni of the steel.

Ac3 is calculated using Thermo-Calc® software.

The steel samples were heated to a temperature of 1000° C. to 1280° C., then hot rolled to a finishing temperature of 850° C. or higher, and then wound at a temperature of 580° C. or lower. The hot rolled coil was cold rolled with a thickness reduction of 30 to 80%. These cold-rolled steel sheets were subjected to heat treatment described later. Then, the steel sheets were hot-dip coated in a zinc bath at a temperature of 460° C. and subjected to batch annealing for 24 hours.

[Table 2a]

[Table 2b]

[Table 2c]

Table 3-Microstructure

Table 3 shows the results of tests performed according to the standard in different microscopes, such as scanning electron microscopy for determining the microstructure composition of the inventive steel and the reference steel.

The result is defined as a percentage of the area, excluding the carbon content of the retained austenite expressed in weight percent. It was observed that all inventive examples had a homogeneous martensite distribution, while all comparative examples had a non-homogeneous distribution.

[Table 3]

Table 4-Mechanical properties

Table 4 illustrates the mechanical properties of the inventive and reference steels. Tensile tests are performed according to the NF EN ISO 6892-1 standard. The hole expansion ratio is measured according to ISO16630:2009, where a sample of 10 punched mm is deformed. After deformation and cracking, the hole diameter is measured and the HER% is calculated as 100*(Df-Di)/Di.

Here the results of various mechanical tests performed according to the standard are summarized in the following table.

[Table 4]

Regarding spot weldability, the sheet according to the invention has a low LME sensitivity when the composition is C + Si/10 ≤ 0.30%. This means that by using such a steel material, it is possible to produce a structure including resistance spot welds such as a vehicle body. In this case, the probability of the number of cracks in the resistance spot weld is less than 5 per resistance spot weld and 10 The probability is less than 98%.

In particular, a welded structure comprising resistance spot welds of at least two steel sheets is C + Si/10 ≤ 0.30% and Al ≥ 6 (C + Mn/10)-2.5% by the method according to the invention and also Zn Alternatively, to prepare a first steel sheet coated with a Zn alloy, and provide a second steel sheet having a composition such that C + Si/10 ≤ 0.30% and Al ≥ 6 (C + Mn/10)-2.5%, and the first steel It can be produced by resistance spot welding the sheet to the second steel sheet. The second steel sheet can be produced, for example, by the method according to the invention and can be coated with Zn or a Zn alloy.

Thus, a welded structure with low LME sensitivity is obtained. For example, for such a welded structure comprising at least 10 resistance spot welds, the average number of cracks per resistance spot weld is less than 5.

The steel sheet selectively welded by resistance spot welding according to the present invention is advantageously used in the manufacture of structural parts of automobiles because it provides high formability during the manufacturing process and high energy absorption in the event of a collision. The resistance spot weld according to the present invention is also advantageously used in the manufacture of structural parts of automobiles because the final initiation and propagation of cracks located in the welding area is much reduced.

Claims (19)

As a coated steel sheet,
The steel sheet contains the following elements expressed in weight percent,
0.17% ≤ carbon ≤ 0.24%,
1.9% ≤ manganese ≤ 2.2%,
0.5% ≤ silicon ≤ 1%,
0.5% ≤ aluminum ≤ 1.2%,
Where Si + Al ≥ 1.3%,
0.05% ≤ chromium ≤ 0.2%,
0.015% ≤ niobium ≤ 0.03%,
Sulfur ≤ 0.004%,
Phosphorus ≤ 0.03%
And, possibly, the following optional elements,
0.005% ≤ titanium ≤ 0.05%,
0.001% ≤ molybdenum ≤ 0.05%
It has a composition comprising at least one of,
The balance consists of iron and inevitable impurities resulting from smelting, and the microstructure of the coated steel sheet is 10 to 20% of retained austenite with a carbon content of 0.9 to 1.1% in the austenite phase, as an area fraction, 40 to 55% of polygonal ferrite, 15 to 40% of granular bainite and 5% or more of tempered martensite, the sum of the tempered martensite and retained austenite is 20 to 30%, the coated steel sheet Does not have untempered martensite, the coated steel sheet has a tensile strength of 980 MPa or more, a yield strength of 550 MPa or more, a ratio of yield strength to tensile strength of 0.60 or more, and a total elongation of 17% or more, and ISO Coated steel sheet with a hole expansion ratio of at least 18% measured according to Standard 16630:2009. The method of claim 1,
The composition is a coated steel sheet, characterized in that it comprises 0.7% ≤ Si ≤ 0.9% by weight. The method according to claim 1 or 2,
The composition is a coated steel sheet, characterized in that it comprises 0.7% ≤ Al ≤ 0.9% by weight. The method according to claim 1 or 2,
Coated steel sheet, characterized in that the sum of silicon and aluminum content exceeds 1.4%. The method according to claim 1 or 2,
Coated steel sheet, characterized in that the carbon and silicon content is C + Si/10 ≤ 0.30%. The method according to claim 1 or 2,
A coated steel sheet, characterized in that the content of aluminum, carbon and manganese is Al ≥ 6 (C + Mn/10)-2.5%. The method according to claim 1 or 2,
Coated steel sheet, characterized in that the sum of retained austenite and tempered martensite is 25% to 30%. The method according to claim 1 or 2,
The tempered martensite fraction (TM) and the average tempered martensite fraction (TM ^* ) measured in an arbitrary area of 50×50 μm ² of the steel sheet are |(TM)-(TM ^* )| Coated steel sheet, characterized in that ≤ 1.5%. delete The method of claim 1,
Coated steel sheet, characterized in that the tensile strength is 1000 MPa to 1100 MPa, and the hole expansion ratio is 18% to 23%. delete The method according to claim 1 or 2,
The coated steel sheet, characterized in that the steel sheet is hot-dip galvanized. As a method for producing a coated steel sheet according to claim 1 or 2,
The following successive steps:
-Providing a semi-finished product of the composition according to claim 1 or 2,
-Reheating the semi-finished product to a temperature of 1000°C to 1280°C,
-Obtaining a hot-rolled steel sheet by completely rolling the semi-finished product in an austenite range having a hot rolling end temperature of 850°C or higher,
-Cooling the hot-rolled steel sheet to a winding temperature of 580°C or less at a cooling rate of 35 to 55°C/s, and winding the hot-rolled steel sheet,
-Cooling the hot-rolled steel sheet to room temperature,
-Pickling the hot-rolled steel sheet,
-Cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet,
-Subsequently, continuously annealing the cold-rolled steel sheet at a soaking temperature of Ac1 to Ac3 for less than 600s at a heating rate of 1 to 20°C/s,
-Then, cooling the sheet to a temperature of 400 to 480°C at a rate of more than 25°C/s, and holding the cold-rolled steel sheet for 20 to 250 seconds,
-Coating the cold-rolled steel sheet by hot dipping in a zinc or zinc alloy bath,
-Cooling the cold-rolled steel sheet to room temperature,
-Subsequently, batch annealing the coated cold-rolled steel sheet to a soaking temperature of 170 to 350°C for 12 to 250 h at a rate of 1 to 20°C/s, and then cooling the sheet to room temperature.
Method for producing a coated steel sheet comprising a. The method of claim 13,
The winding temperature is a method of manufacturing a coated steel sheet, characterized in that lower than the bainite transformation initiation temperature Bs. The method of claim 13,
The cracking temperature is 780 ℃ to 900 ℃, the method for producing a coated steel sheet, characterized in that the cracking is carried out for 10 to 600s. The method of claim 13,
The sheet is a method for producing a coated steel sheet, characterized in that the sheet is cooled to a temperature of 400 to 480°C at a cooling rate of more than 30°C/s after continuous annealing. The method of claim 16,
The method for producing a coated steel sheet, wherein the steel sheet is coated in a zinc or zinc alloy bath and then cooled at a cooling rate of less than 20°C/s. The method of claim 13,
The steel sheet is a method for producing a coated steel sheet, characterized in that the batch anneal for 12 to 30h at 170 ℃ to 250 ℃. delete

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